In the transmission of large volumes of gas, such as natural gas for example, a pipeline system is typically employed having compressor stations located at intervals along the length thereof to increase the pressure of the flowing gas and thus compensate for pressure loss due to line friction and other factors. The least amount of horsepower required to move a given quantity of gas between compressor stations occurs when the stations are connected by a single large gas transmission pipeline. Since natural gas is typically shipped in large volumes the physical dimension of a single large diameter pipeline to transport the required gas volume would be impractical from a commercial standpoint. The next most efficient method for transporting that same volume of gas or fluid is to connect the compressor stations with two or more gas transmission lines having equalizing lines interconnecting them at periodic intervals. For this reason most interstate gas transmission networks incorporate plural parallel main lines which form parallel looped transmission networks having equalizing lines interconnecting the main lines at periodic intervals and thus maintaining substantially equal pressure in all of the main lines. The main lines have valves located at periodic intervals along the length thereof and the equalizing lines also typically incorporate valves so that the main lines can be isolated, if desired. The valves of the main lines and equalizing lines may be selectively closed to isolate portions of the gas transmission network in the event service or repair is required without necessitating shutting down of the entire gas transmission network.
When a gas transmission network of this nature is in full operation all of the main line valves and equalizing line valves are typically open and the pressure of the gas therein is substantially equal at any particular point along the length thereof because of the presence of the equalizing lines. When a gas transmission network is operating in this mode, the fluid flow in the equalizing lines is either very low or non-existent. If a pressure variation occurs in one of the main lines, it will typically be maintained as it flows along the main line until it reaches an equalizing line. This pressure variation will then be communicated by the equalizing line to each of the other main lines, thus ensuring that as the pressurized gas reaches a subsequent compressor station the pressure in each of the main lines will be substantially equal.
When a pressure variation in a main line is equalized by an equalizing line, obviously there is flow in the equalizing line from high pressure to low pressure. Flow therefore occurs in the equalizing lines to accomplish pressure balancing. The direction and duration of flow in the equalizing line is of course responsive to the magnitude of the pressure variation that has occurred. Typically, the flow in an equalizing line is of low velocity and short duration except under circumstances where a line rupture occurs. If a main line should rupture, the flow in the equalizing line rises rapidly due to the unbalanced pressure condition between the ruptured and unruptured lines. Additionally, the direction of flow in the equalizing lines is toward the rupture. It is these two parameters, magnitude of flow and direction of flow, which form the basis for the control logic of the present invention. Obviously, it is necessary that a line break detection and isolation system be capable of efficiently distinguishing between short term line pressure surges and actual line rupture to thereby ensure against automatic shut down of the gas transmission system in the event short term pressure surges are encountered.